1. Field of Invention
The present invention relates to a surface modification of a porous ceramic substrate and the fabrication of hydrogen-permeable composite palladium or palladium alloy membranes.
2. Related Art
Palladium membranes (including palladium-alloy membranes) are highly and exclusively permeable to hydrogen, and they have been applied as hydrogen separators or purifiers. In addition, they are stable at high temperature, which enables extensive applications. The membrane thickness can be greatly decreased once it is supported on a porous substrate. Such a concept of the “composite membrane” not only saves the precious metal but also increases the hydrogen permeability, which is inversely proportional to the membrane thickness. The substrate materials are mainly porous ceramics and porous stainless steels, but the former are predominant because of their superior chemical stability and the availability on the market. Among the various techniques for palladium membrane fabrication, electroless plating is one of the best. Before plating, the substrate surface must be activated by seeding metal nuclei as catalysts [Y. Huang, X. Li, Y. Fan, N. Xu. Palladium-based composite membranes: Principle, preparation and characterization. Prog. Chem., 2006, 18(2-3): 230-238.]; [J. Yu, X. Hu, Y. Huang. Ceramic modification of the porous stainless-steel surface toward the supported palladium membranes for hydrogen separation. Prog. Chem., 2008, 20(7-8): 1208-1215.].
The cost of the composite palladium membranes is a difficult problem against their more extensive applications. In addition to precious palladium, the substrate materials are also expensive. Although various porous ceramics are available on the market, they are mainly used as filters for solid-liquid separation rather than as membrane substrates. As far as the solid-liquid filtration is concerned, several defects (such as cracks and holes) in the filter may not be a large problem because of the formation of filter-cake, whereas, when the filter is used as a palladium membrane substrate, these defects can eventually lead to poor membrane selectivity or even a membrane failure. It is well known that the surface pore size and roughness of the porous ceramics strongly influence the fabrication of the palladium membranes, and, more precisely, porous substrates with a smooth surface and small pore size help to achieve thin and defect-free palladium membranes. However, this kind of porous ceramic often has an asymmetric structure and is extremely expensive, leading to a high cost for the whole membrane. If ordinary porous ceramic substrates are employed instead, a surface modification pretreatment is necessary to improve the surface properties, and a routine operation is surface coating.
The common coating materials are Al2O3, ZrO2, SiO2, MgO, TiO2, etc., and the most popular coating technique is the sol-gel process, which comprises (i) the preparation of a sol, (ii) coating the substrate, (iii) drying to form a gel layer, and (iv) a heat treatment to decompose the gel layer and to form a thin porous ceramic layer [X. L. Zhang, G. X. Xiong, W. S. Yang. A modified electroless plating technique for thin dense palladium composite membranes with enhanced stability. J. Membr. Sci., 2008, 314: 226-237.]; [H. B. Zhao, K. Pflanz, J. H. Gu. Preparation of palladium composite membranes by modified electroless plating procedure. J. Membr. Sci., 1998, 142: 147-157.]. However, the sol-gel process may be complicated and expensive, and it suffers from problems such as cracking and peeling at the coating layer. Moreover, it may be unsuitable to apply directly on macroporous ceramics. Xu et al. [H. Xu, S. Hou, W. Li, K. Jiang, L. Yuan. Composite metal palladium membrane or alloy palladium membrane and preparation process thereof. CN. Pat., No. 200410021025.6, 2004] patented a modified sol-gel process, in which the resulting gel layer on the porous ceramic substrate is not decomposed until the palladium membrane is deposited via electroless plating. However, the gel layer is unstable and may be damaged during the electroless plating, when the substrate has to go through acidic and basic solutions (the acidic solution is for activation pretreatment, and the basic solution is the plating bath, which also contains chelating agents at high concentration). Moreover, the final decomposition of the gel during heat treatment may lead to a poor membrane adhesion.